Aromatic polyimides have found extensive use in industry as fibers, composites, molded parts and dielectrics due to their toughness, flexibility, mechanical strength and high thermal stability. In the electronic industry, polyimides have proven to be useful due to their low dielectric constant and high electrical resistivity. Such polymers have been used in both film and coating form as advanced materials for such uses as interlevel dielectrics, passivation coatings, insulating coatings, optical waveguides, planarization layers, die attach adhesives, solder masks, flexible circuit substrates, fabrication aids, and the like.
Many electronic applications, for example, passivation coatings on a substrate, require that openings such as vias or patterns be etched through the polymer coatings to permit access for electrical connections that run between the substrate and the outside environment. There are two conventional methods for etching these polymer coatings: dry etching and wet etching. Dry etching generally involves the use of gaseous plasmas of oxygen or oxygen/carbon tetrafluoride mixtures which are exposed to a fully cured polyimide. The dry-etching process, however, is slow and requires sophisticated equipment.
Wet-etching processes differ depending on the state of cure (or imidization) of the polyimide. Fully or substantially fully cured polyimides can be etched using either hydrazine hydrate or ethylene diamine. The use of these compounds is not preferred for various reasons. Wet etching of partially cured polyamic acid films typically involves the use of basic solutions, for example, aqueous solutions of either tetramethylammonium hydroxide (TMAH) or sodium hydroxide (NaOH). However, aqueous solutions of TMAH or NaOH do not completely etch many of the recently developed polyimides.
Recently, polyimides have been developed that have improved dielectric constants and lower moisture uptake values. One approach to developing the improved polyimides has been to dilute the amic acid content of the polymer by using longer chain starting materials. However, since it is the amic acid moiety that forms a soluble salt with the hydroxide moiety of the aqueous hydroxide solution, diluting the amic acid moiety contributes to a decrease in solubility. Another approach has been to introduce non-polar (hydrophobic) substituents such as fluorine into the polyimide structure. Fluorinated polyimides offer up to 20 percent reduction in dielectric constant, indicating a less polar polymer and higher signal speeds as compared to conventional polar polyimides. In addition, fluorinated polyimides absorb less than one-third of the moisture that is absorbed by many conventional polyimides. Although the increased hydrophobic nature of these polyimides produces a polymer with the desirable properties, it also makes these polyimides more difficult to process by the usual wet chemical techniques due to their lower solubility in aqueous solutions.
Ruiz et al., Electronic Materials and Processes, International SAMPE Electronics Conf. Series, Vol. 3 (1989), examined the performance properties of fluorinated polyimides and concluded that patterning must be carried out using oxygen plasma (dry) etching techniques.
Additionally, during the wet-etch procedure, a pattern of positive photoresist material remains on the polyamic acid layer to cover and protect selected areas of polyamic acid and provide a means of etching (or removing) only the areas of the polyamic acid coating not covered by photoresist material. Accordingly, care must be taken such that the wet-etch composition does not attack the photoresist material, otherwise the entire layer of polyamic acid may be removed, and the desired vias through the polymer coating will not be created.
Davis et al., Recent Advances in Polyimide Science and Technology, Proc. 2nd Ellenville Conf. on Polyimides, 381-388 (1987), investigated a 3,3'-4,4'-benzophenonetetracarboxylic acid dianhydride, 4,4'-methylenedianiline, polytetramethyldisiloxane diamine-derived polymer system. The reference teaches that wet-etch compositions containing sodium hydroxide, potassium hydroxide, or tetramethylammonium hydroxide can lead to significant undercut of the photoresist material.
Attempts have been made to provide wet-etch solutions for polyimides and polyamic acids. U.S. Pat. No. 4,857,143 is directed to the wet-etching of fully cured or substantially fully cured polyimides. This references discloses an aqueous solution of a metal hydroxide such as an alkali metal or alkali earth hydroxide and requires the presence of a metallic compound selected from metal carbonates, sulfates and phosphates.
U.S. Pat. No. 4,411,735 discloses an etchant composition for etching a partially cured polyimide or polyimide-isoindoloquinazolinedione comprising a water solution containing an amine etchant or, alternatively, a water solution containing a mixture of a quaternary ammonium hydroxide and an N-alkyl pyrrolidone.
U.S. Pat. No. 4,740,562 discloses an etching solution for modifying a polymer surface. This reference attempts to affect the anti-adhesive behavior and low adhesion of the surface of objects made from polyvinylidene fluoride so that composite structures and laminates can be made from the polymer. Dehydrofluorination is carried out with an etching solution containing a strongly basic compound (hydroxides and/or alcoholates of alkali, alkaline earth, or earth metals), a swelling agent and/or solvent for polyvinylidene fluoride, and an auxiliary agent for the homogenization of the etching solution. The auxiliary agent, which also acts as a solubilizer for the basic compound, includes methanol, ethanol, isopropanol, cyclohexanol, ethylene glycol and/or glycerin.
Diener et al., Polyimides, Vol. 2, Plenum Press, 353-364 (1984), teach that partially cured polyimide films made from pyromellitic dianhydride and 4,4'-oxydianiline can be wet etched with potassium hydroxide.
Saiki et al., Polyimides, Vol. 2, Plenum Press, 827-839 (1984), teach that an alcohol solution of tetramethylammonium hydroxide is the most suitable etchant for a polyimide reported as polyimide isoindoloquinazolinedione (PIQ). However, high levels of alcohol in the presence of hydroxide etchants can dissolve the photoresist material. Further, this reference also teaches that an aqueous solution of tetramethylammonium hydroxide has no potential for future etching processes.
U.S. Pat. No. 4,276,186 discloses a cleaning composition which includes N-methyl-2-pyrrolidone (NMP) and an alkanolamine for removing solder flux from a substrate. This reference is directed to stripping (or completely removing) a particular material from a substrate. On the other hand, the present invention is directed to selectively removing predetermined areas of polyamic acid coating.
U.S. Pat. No. 4,592,787 discloses a composition for stripping cross-linked photoresist material (i.e. a negative photoresist material). The present invention is intended to avoid stripping unexposed positive photoresist material while, at the same time, selectively removing the polyamic acid.
While these references disclose wet-etch compositions for polyimides, there is no disclosure of the wet-etch composition of the present invention and no teaching that such a wet-etch composition will etch polyimides that are not completely etched in the usual ionic base wet-etch compositions.
Accordingly, it is an object of the present invention to provide an improved wet-etch process and a composition therefor. This and other objects and advantages of the present invention will be apparent to those skilled in the art from the following description.